1. Field of the Invention
The present invention generally relates to an electrode of an oxygen-ion conductive cell used in sensors for detecting gas components such as NO.sub.x, CO.sub.2, SO.sub.x, HC and H.sub.2 O, such as a NO.sub.x --concentration sensor used for detecting the concentration of nitrogen oxides contained, e.g., in an exhaust gas from a stationary internal combustion engine for industrial use or an internal combustion engine for an automobile, ship, or airplane, or a combustion gas of a boiler or the like. More particularly, the present invention relates to an electrode provided in a first measurement chamber among a pair of electrodes of a first oxygen-ion pump cell of the sensor for measuring NO.sub.x concentration, to a method of measuring NO.sub.x -concentration using said sensor, and to a method for manufacturing the electrode provide in the first measurement chamber of the sensor.
2. Description of Related Art
In order to cope with increasingly strict regulation of exhaust gas, there has been studied a technique for directly measuring gas components in exhaust gas from an engine in order to control the engine or a catalyst device. Especially, because of the capability of measuring NO.sub.x concentration without influence of interference gases such as HC and CO, there has been widely studied an NO.sub.x -concentration sensor which utilizes an oxygen-ion conductor such as ZrO.sub.2 and in which oxygen is pumped out through a first oxygen-ion pump cell to such an extent that NO.sub.x substantially does not decompose, NO.sub.x contained in the remaining gas is decomposed through use of a second oxygen-ion pump cell, and the decomposition of the NO.sub.x is detected in the form of current.
For example, Japanese Patent Application Laid-Open No. 8-271476 which corresponds to U.S. Pat. No. 5,672,811 discloses a method of measuring a gas component and a sensing device for measuring the gas component. In this device, the electrode that constitutes the first oxygen pump means and is disposed within the first space is a cermet electrode formed of a mixture of Pt and ZrO.sub.2 or a cermet electrode formed of a Pt/Au alloy. The method of forming the Pt/Au-alloy cermet electrode or the microstructure of the cermet is not described in this prior art. However, it is supposedly generally formed as follows. That is, Pt powder, Au powder, ZrO.sub.2 powder, and a proper amount of organic solvent may be mixed and dispersed to obtain a mixture, to which may be added an organic binder. Subsequently, a viscosity modifier may be further added to and mixed with the mixture to obtain paste. The paste may be then applied onto a solid electrolyte layer and baked. As shown in FIG. 2, the thus-supposedly formed electrode will have a texture such that precious-metal alloy particles and ZrO.sub.2 particles exist independently of each other.
In the case of NO.sub.x measurement, the above-described first oxygen pumping means has a function of selectively pumping out oxygen without dissociating NO.sub.x. However, when the method of simply adding Au powder to an electrode constituent component is employed, Au and Pt do not form an alloy if the ratio of Au exceeds a certain value, so that Au unevenly disperses within the electrode (see FIG. 2). In such a case, the function of suppressing NO.sub.x dissociation unstably varies with time and also varies locally, and therefore, the function of suppressing NO.sub.x dissociation is difficult to improve. Further, since the function of the Au component for suppressing NO.sub.x dissociation is unstable, the oxygen concentration within the first internal space must be set slightly higher than a theoretical level, in consideration of variation in the suppressing function (because NO.sub.x dissociates when the oxygen concentration decreases). The excessive oxygen causes an error when a very small NO.sub.x concentration is measured.
In view of the foregoing, an object of the present invention is to provide an electrode which is provided in a first measurement chamber of a NO.sub.x -concentration sensor as one of the paired electrodes of a first oxygen-ion pump cell, which has an enhanced function of suppressing NO.sub.x dissociation, and which stably provides the function of suppressing NO.sub.x dissociation.